An ultrasonic diagnostic apparatus obtains biological information from a subject by irradiating him or her with an ultrasonic wave and analyzing the information contained in its echo signal. For example, a conventional ultrasonic diagnostic apparatus that has been used extensively converts the intensity of the echo signal into its associated pixel luminance, thereby presenting the subject's internal structure as a tomographic image. In this manner, the internal structure of the subject can be known.
Some people are attempting recently to track the motion of a subject's tissue and evaluate the strain and the elasticity, viscosity or any other attribute property of the tissue mainly by analyzing the amplitude or phase information of the echo signal. For example, Patent Document No. 1 and Non-Patent Document No. 1 disclose a method for obtaining the magnitude of strain based on the magnitude of displacement of a measuring point that has been set on the vascular wall due to the heartbeat and calculating the local elasticity around the measuring point based on the magnitude of the strain and the blood pressure difference and also disclose a method for presenting the spatial distribution of elasticities as an image. Hereinafter, these methods will be described.
The arterial vascular wall will be deformed due to a variation in blood pressure that has been caused by the heartbeat. Based on the relation between the degree of that deformation (i.e., the magnitude of strain) and the stress produced by the blood pressure in the arterial vascular wall, the elasticity of the vascular wall can be defined. In this case, it is difficult to measure noninvasively, or estimate indirectly, the distribution of stress in the arterial vascular wall. For that reason, the magnitude of strain ε of the arterial vascular wall during one heartbeat is measured with ultrasonic waves. And based on the difference between the lowest and highest blood pressures Pd and Ps that have been measured separately with a blood pressure manometer (i.e., based on the pulse ΔP=Ps−Pd), the elasticity E of the arterial vascular wall is defined by the following Equation (1):
                    E        =                              Δ            ⁢                                                  ⁢            P                    ɛ                                    (        1        )            
Using ultrasonic waves, the magnitude of strain may be measured in the following manner:
First of all, points A and B are set on the intima and adventitia of the vascular wall of the artery 502 of the subject 503 as shown in FIG. 12(a). Suppose an ultrasonic beam 504 has been sent from a probe 501 to obtain an echo signal and the magnitude of strain ε of the vascular wall between those two points is calculated based on that echo signal.
The positions of those two points at a point in time t are identified by XA(t) and XB(t), respectively, and a time t==0 is set using the R wave of the electrocardiogram at the end of the diastole as a trigger. FIG. 12(b) shows exemplary waveforms of XA(t) and XB(t). After t=0, XA(t) and XB(t) both decrease because the blood vessel shrinks.
The initial thickness h0 is calculated by h0==XB(0)−XA(0) and the greatest thickness difference Δh is represented by the following Equation (2):Δh=MAX[|XB(t)−XA(t)|]  (2)
The magnitude of strain ε is given by the following Equation (3):
                    ɛ        =                                            Δ              ⁢                                                          ⁢              h                                      h              0                                =                                    MAX              ⁡                              [                                                                                              XB                      ⁡                                              (                        t                        )                                                              -                                          XA                      ⁡                                              (                        t                        )                                                                                                              ]                                                                    XB                ⁡                                  (                  0                  )                                            -                              XA                ⁡                                  (                  0                  )                                                                                        (        3        )            where MAX [*] is a function representing the maximum value of *.
Generally speaking, the displacement of the arterial vascular wall due to the dilation or shrinkage of the artery is on the order of several hundred μm, while the greatest thickness difference Δh of the vascular wall is on the order of several ten μm. Thus, the variation in thickness, which is approximately one digit smaller than the wavelength of the ultrasonic waves for use to make measurement (e.g., about 300 μm), should be sensed accurately.
In the example described above, the magnitude of strain ε is supposed to be calculated between two points on the intima and adventitia sides of the vascular wall. On the other hand, according to the methods disclosed in Patent Document No. 1 and Non-Patent Document No. 1, a number of displacement measuring points are set at intervals of approximately 80 μm on each ultrasonic beam 504 within the measuring range 510 and the magnitude of displacement is measured at each of those measuring points as shown in FIG. 13. The ultrasonic wave for use to make measurement has a pulse width of approximately 400 μm. That is why two measuring points are set so that the initial thickness h0 calculated by Equation (1) becomes approximately 400 μm, the greatest thickness difference Δh is calculated by Equation (2) on the supposition that the thickness variation is constant between those two points, and Δh is supposed to be the greatest thickness difference at the midpoint between those two points. While vertically shifting that layer from the intima side of the vascular wall toward its adventitia side by the interval between each pair of measuring points each time, Δh is calculated with respect to each measuring point. Furthermore, by scanning the blood vessel 502 with the ultrasonic beam 504 along its length at intervals of several hundred μm, thousands of very small regions are defied in the axial and depth directions of the vascular wall and Δh is calculated in each of those very small regions. The magnitude of strain ε in each very small region is calculated based on h and Δh thus obtained, and the elasticity E of each very small region 511 is obtained by Equation (1) based on the pulse ΔP that has been measured separately with a blood pressure manometer.